1. Field of the Invention
The present invention relates generally to tempered glass and, more particularly, to a system and method for simultaneously heating and cooling glass to produce tempered glass.
2. Description of the Related Art
Tempered or heat-treated glass is generally defined as glass (e.g., annealed or ordinary) that has been pre-stressed by being heated to a temperature substantially at or above the glass's softening point and being forced to suddenly and rapidly quench under carefully controlled conditions. The tempering process produces tempered glass having highly desirable conditions of induced stress that result in additional strength, resistance to thermal stress, and impact resistance as compared to annealed or ordinary glass.
The basic principle employed in the tempering process is to create an initial condition of glass-surface and -edge compression. This condition is achieved by first heating the glass and then quenching the surfaces of the glass rapidly. Such heating and quenching leaves the center thickness of the glass hot relative to the surfaces of the glass. As the center thickness cools, the surfaces are forced into compression. Wind pressure, missile impact, thermal stresses, or other applied loads must first overcome this compression before there is any possibility of fracture of the glass.
With respect to the heating step, it is known to use a hearth or lehr to heat glass sheets that are to be tempered. Generally speaking, a lehr is a furnace and may be of a continuous roller-type, fixtured roller-type, or gas-type. For example, a gas-type lehr has a plurality of blocks disposed beneath a plurality of radiant heaters. Typically, a glass sheet is placed inside the lehr where the glass sheet is heated by conventional radiation, convection, and conduction heat. The glass sheet is moved along the blocks at a predetermined rate, which depends upon the thermal conductivity of the glass sheet, to reach a temperature in the glass sheet's forming range. When such a temperature is reached (e.g., approximately 120° F.), the glass sheet is formed to a predetermined shape of the blocks.
Once formed, the glass sheet is rapidly air-quenched, typically by application of an air stream to the glass sheet. The air stream can consist of arrays of fixed, reciprocating, or rotating nozzles. It is important to extract heat uniformly from both surfaces of the glass sheet (uneven heat extraction may produce bow or warp) and to sustain the quench long enough to prevent reheating of the glass surfaces from the still-hot center of the glass sheet. A quenched condition becomes stable when the glass sheet is reduced to a temperature of approximately 400° F. to 600° F.
Although the above-described lehr works well, it suffers from the disadvantage that the lehr must be long enough in length to allow the glass sheet to be heated at the predetermined rate. This length requires a large quantity of floor space, energy consumption, and cost.
A recent approach to overcoming this disadvantage is to employ microwave energy [at frequencies in the range of 2 gigahertz (GHz) to 40 GHz] to rapidly and efficiently heat a glass sheet that has been pre-heated to a temperature substantially at or above its softening temperature by conventional means. This approach is more fully described in U.S. Pat. Nos. 5,782,947 and 5,827,345 to Boaz, the disclosures of which are hereby incorporated by reference.
U.S. Pat. No. 5,782,947 to Boaz discloses a method for heating a glass sheet including the steps of heating the glass sheet to a first predetermined temperature and applying microwave energy to the glass sheet to heat it to at least a second predetermined temperature to allow the glass sheet to be formed. One advantage of the method described in U.S. Pat. No. 5,782,947 to Boaz is that the length of the lehr is reduced, which results in less floor space and increased throughput (speed and yield) of the glass sheet that is formed.
U.S. Pat. No. 5,827,345 to Boaz discloses a method for heating, forming, and tempering a glass sheet including the steps of heating the glass sheet to at least a first predetermined temperature, applying microwave energy to the glass sheet to heat it to at least a second predetermined temperature, forming the glass sheet to a predetermined configuration, and cooling at least one outer surface of the glass sheet to at least a third predetermined temperature to temper the glass sheet. One advantage of the method described in U.S. Pat. No. 5,827,345 to Boaz is that a relatively thin glass sheet (e.g., less than 0.125 inch in thickness) can be tempered. More specifically, while the center of the glass sheet is being heated by the microwave energy, the outer surfaces of the glass sheet are being cooled, thus creating a desired temperature differential or gradient between the center and the outer surfaces of the glass sheet.
Although the methods described in U.S. Pat. Nos. 5,782,947 and 5,827,345 to Boaz represent significant advances in glass-tempering technology, these methods suffer from the disadvantage that the disclosed microwave-energy levels (i.e., having a frequency range of 2 GH to 40 GH) are relatively expensive to generate and maintain over an extended production period. Additionally, the use of such high frequency microwave energy levels presents operational problems in a conventional production facility setting. Therefore, there is a need in the art for a system and method for rapidly, efficiently, and inexpensively heating glass during the heating portion of the tempering process while maintaining a desired temperature differential or gradient between the center of the glass sheet and the outer surfaces of the glass sheet to facilitate the production of tempered glass, especially relatively thin tempered glass.